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DNA Introduction

Blueprints contain the instructions for building a house. Your cells also contain “blueprints” known as DNA, or deoxyribonucleic acid. DNA must do two things: 1. supply instructions for cell processes and the building cell structures. 2. be able to be copied each time a cell divides so that each cell contains an

identical set of genes.

The Structure of DNA

DNA is made of smaller pieces called nucleotides. Each nucleotide is made

of three parts: a sugar, a phosphate, and a nitrogenous (nitrogen) base. Nucleotides are identical except for the bases, which come in one of four varieties – adenine (A), thymine (T), guanine (G), or cytosine (C). In the 1950’s, a biochemist named Erwin Chargaff found that the amount of adenine in a DNA strand always equals the amount of thymine and that the amount of guanine always equals the amount of cytosine. From this, he concluded that the nucleotides with particular bases must pair together. We abbreviate Chargaff’s rules like this: A = T and G = C. Rosalind Franklin, who was the first person to create images of DNA molecules, made the next contribution to the study of DNA. She used a process called X-ray diffraction to discover that DNA had a spiral shape. Two other young scientists expanded on Franklin’s discovery – James Watson and Francis Crick built models of DNA and concluded that DNA resembles a twisted ladder, which they called a double helix.

From DNA to Gene Each cell has 46 chromosomes, which are made of

DNA and protein. A section of a chromosome for a particular trait, such as height, is called a gene. A gene may contain thousands of loops of DNA. The loops are made of the DNA ladder, which looks like a spiral staircase.

Making Copies of DNA

Because A always binds with T and G always binds with C, the two sides of the ladder are said to be complementary to the other. This allows DNA to make a copy of itself, or replicate. When this occurs, the DNA molecule splits (unzips) down the middle where the two bases meet. The existing bases are used as the pattern for the complementary side.

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How DNA Works

1. A copy of a portion of the DNA molecule where a particular gene is located is made and transferred outside of a cell’s nucleus.

2. This single strand is a copy of one strand of the original DNA. 3. Each group of three bases codes for one amino acid. 4. A copy of the DNA strand is fed through the ribosome. 5. Transfer molecules deliver amino acids from the cytoplasm of the cell to the

ribosome. 6. The amino acids are dropped off at the ribosome. 7. The amino acids join together to make a protein. 8. The order of the bases on the copy of DNA determines which amino acids are

transferred to the ribosome and which protein is eventually made.

Changes In Genes

Damage to DNA can be caused by physical and chemical changes. A mutagen is anything that causes a

mutation in DNA. Mutagens can be UV light, x-rays, asbestos, cigarette smoke, and many others. Mutations occur when there is a change in the order of bases in an organism’s DNA. When a base is left out, it is called deletion. When a base is added, it is called insertion. The most common error occurs when an incorrect base replaces a correct base. This is called substitution. Fortunately, repair enzymes are continuously on the job, patrolling the DNA for errors. Usually the errors are repaired, but sometimes the mistake becomes part of the genetic message. These changes can be either helpful, harmful, or have no consequence.

Review of “DNA” Reading

1. What must DNA be able to do for an organism?

2. Describe the smaller parts of DNA and how they fit together.

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3. Describe the contributions of scientists to the study of DNA.

4. What is the relationship between DNA, chromosomes, and genes?

5. Describe the types of mutation.

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DNA Hierarchy Notes

Are four types of...

These come together to make...

Amounts Amounts

Word Bank

Adenine

Bases

Chromosomes

Cytosine

DNA

Genes

Guanine

Nucleotides

Phosphates

Sugars

Thymine

...are segments of...

...are the building blocks of...

...coils up to make...

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DNA – The Double Helix Recall that the nucleus is a small spherical, dense body in a cell. It is often called the "control center"

because it controls all the activities of the cell including cell reproduction, and heredity. Chromosomes are microscopic, threadlike strands composed of the chemical DNA (short for deoxyribonucleic acid). In simple terms, DNA controls the production of proteins within the cell. These proteins form the structural units of cells and control all chemical processes within the cell. Think of proteins as the building blocks for an organism, because proteins make up your skin, your hair, and parts of the cell membrane. How you look is largely determined by the proteins that are made, which is determined by the sequence of DNA in the nucleus.

Chromosomes are composed of genes, which are segments of DNA that code for a particular protein

which in turn codes for a trait. Hence you hear it commonly referred to as the gene for baldness or the gene for blue eyes. Meanwhile, DNA is the chemical that genes and chromosomes are made of. DNA is called a nucleic acid because it was first found in the nucleus. We now know that DNA is also found in organelles, the mitochondria and chloroplasts, though it is the DNA in the nucleus that actually controls the cell's workings.

In 1953, James Watson and Francis Crick established the structure of DNA. The shape of DNA is a

double helix, which is like a twisted ladder. The DNA helix is actually made of repeating units called nucleotides. Each nucleotide consists of three molecules: a sugar (deoxyribose), a phosphate, and then one of the four nitrogenous bases. The sides of the ladder are made of alternating sugar and phosphate molecules. The sugar in DNA is deoxyribose.

The rungs of the ladder are pairs of 4 types of nitrogen bases. The bases are known by their coded

letters: A, G, T, and C. These bases always bond in a certain way. Adenine will only bond to thymine. Guanine will only bond with cytosine. This is known as the "Base-Pair Rule" or “Chargaff’s Rule”. The bases can occur in any order along a strand of DNA. The order of these bases is the code that contains the instructions. For instance ATGCACATA would code for a different gene than AATTACGGA. A strand of DNA contains millions of bases. (For simplicity, the image only contains a few.)

The two sides of the DNA ladder are held together loosely by hydrogen bonds. The DNA can actually "unzip" when it needs to replicate - or make a copy of itself. DNA needs to copy itself when a cell divides, so that the new cells each contain a copy of the DNA. Without these instructions, the new cells wouldn't have the correct information. The hydrogen bonds are represented by small circles.

The Blueprint of Life

Every cell in your body has the same "blueprint" or the same DNA. Like the blueprints of a house tell the builders how to construct a house, the DNA "blueprint" tells the cell how to build the organism. Yet, how can a heart be so different from a brain if all the cells contain the same instructions? Although much work remains in genetics, it has become apparent that a cell has the ability to turn off most genes and only work with the genes necessary to do a job. We also know that a lot of DNA apparently is nonsense and codes for nothing. These regions of DNA that do not code for proteins are called "introns", or sometimes "junk DNA". The sections of DNA that do actually code from proteins are called "exons".

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Questions:

1. Write out the full name for DNA. _____________________________________________________________

2. What is a gene? __________________________________________________________________________

3. Where in the cell are chromosomes located? ___________________________________________________

4. DNA can be found in what two organelles? _____________________________________________________

5. Which two scientists established the structure of DNA? ___________________________________________

6. What is the shape of DNA? __________________________________________________________________

7. What are the sides, or “rails,” of the DNA double helix made of? ___________________________________

__________________________________________________________________________________________

8. What are the "rungs" of the DNA double helix made of? __________________________________________

__________________________________________________________________________________________

9. What sugar is found in DNA? ____________________________

10. How do the bases bond together in DNA? A bonds with _____ and G bonds with _____

11. DNA is made of repeating units called ____________________________________________

12. How do some cells become brain cells and others become skin cells, when the DNA in ALL the cells is exactly the same. In other words, if the instructions are exactly the same, how does one cell become a brain cell and another a skin cell?

13. Why is DNA called the "Blueprint of Life"?

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Coloring:

Color all the phosphates LIGHT BLUE (one is labeled with a "P").

Color all the deoxyribose sugars DARK BLUE (one is labeled with a "D").

Color the thymine bases RED.

Color the adenine bases GREEN.

Color the guanine bases PURPLE.

Color the cytosine bases YELLOW.

Color the hydrogen bonds grey.

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Mutation Practice A mutation is a change in DNA sequence. This can be caused by a mutagen, anything that causes a mutation, or by an error in DNA replication. Mutations usually do not cause any visible change in an organism. While there is a change in DNA sequence, there may not be a change in the function of the protein made by the sequence. There are three types of mutations:

Insertion: An extra base is added to the DNA sequence.

Deletion: A base is removed from the DNA sequence.

Substitution: A base is switched with another base. If a complementary base is switched with its partner, there is no change in the function.

1. Below is half a section of DNA that has been split apart and is ready to copy itself. Write the appropriate letter in the space provided to build the DNA’s new complementary strand.

G........___ T........___ A........___ A........___ C........___ T........___ C........___ C........___ T........___

2. Below are two DNA sequences, the original and the mutated strands. Identify where the mutation occurred and also the type of mutation it is.

C G T A C G C G T A A T A T A T C G C G T A

Base Sequence in Original Cell DNA

C G T A C G C G T A A T A T A T T A C G T A

Base Sequence in Mutated Cell DNA

TYPE OF MUTATION

____________________

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3. Below are two DNA sequences, the original and the mutated strands. Identify where the mutation occurred and also the type of mutation it is. 4. Ribosomes “read” a complementary copy of DNA in order to make proteins. Each group of three bases (called a “triplet” or “codon”) forms the code for an amino acid. When mutations occur, they can change the information that the DNA carries. To understand this process better, look at the sentence below, which uses only three-letter words. AMY GOT THE RED HOT POT OFF THE LOG If one letter is deleted from this sentence, it can become: AMY GTT HER EDH OTP OTO FFT HEL OG How is this similar to what can happen when a mutation occurs in DNA? __________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

C G T A C G C G G C T A A T A T G C A T T A

Base Sequence in Original Cell DNA

C G T A C G C G G C T A A T A T A T T A

Base Sequence in Mutated Cell DNA

TYPE OF MUTATION

____________________

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Lab: DNA & Proteins Investigating Mutations

Objective: Study how mutations in DNA may or may not lead to changes in a trait’s structure or function.

Instructions:

1. For each spoon, transfer water from one container to the other by dipping it into the water, lifting, and pouring the remainder into the empty bucket. One trial = 20 transfers of water.

2. For each trial, record the amount (in milliliters) that has successfully been transferred (round to the nearest tenth) by using the graduated cylinder provided.

Warm-Up Questions:

1. What is DNA? __________________________________________________________________________

2. What is a mutation? _____________________________________________________________________

3. List and describe the three different types of mutations. ________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

4. What is the difference between a trait’s structure and a trait’s function? ___________________________

_______________________________________________________________________________________

Variables: Independent Variable – Dependent Variable –

Hypotheses:

1. If a mutation in DNA occurs, then the structure of the trait will (sometimes, always, never) change. 2. If a mutation in DNA occurs, then the function of the trait will (sometimes, always, never) change.

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Pre-Lab Practice: Study the table below. Circle where the mutation occurs in each of the Mutants, identify which type of mutation has occurred, and indicate whether there is a change in structure (using pictures of the hands).

Protein DNA Strand Type of Mutation Is there a change

in structure?

Normal A C G C G A T C G C G T A C A G T C None None

Mutant A A C G C G A T A C G C G T A C A G T C

Mutant B A C G C G A T C G C G T C C A G T C

Mutant C A G G C G A T C G C G T A C A G T C

Mutant D A C G C A T C G C G T A C A G T C

How It Works:

A mutation changes a ___________ in a DNA sequence. Each set of three bases, called a

____________, codes for a particular _______________ __________. When strung together, the amino acids

code for a particular ______________, which determines the ____________________ of your traits. So, a

change in the protein structure may or may not be visible and may or may not cause a change in

______________________, or how well the trait works.

Data Table:

Protein Type of

Mutation Volume (mL) Transferred

– YOUR SPOON Volume (mL) Transferred

– CLASS AVERAGE Change from Normal

Normal None None

Mutant A Insertion

Mutant B Substitution

Mutant C Substitution

Mutant D Deletion

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Quick Review:

A change in DNA sequence is called a mutation. A change in structure means that the strand of DNA, once paired with its other half, will be different from the original DNA and cause the trait to be “built” differently. If there is a change in function, it means the trait will “work” in a different way than the original trait.

Protein Was there a change in

the DNA sequence? Was there a change in structure

(appearance)? Was there a change in function (amount of water transferred)?

Mutant A Yes

Mutant B Yes

Mutant C Yes

Mutant D Yes